1. Field of the Invention
The present invention relates to an image scanning apparatus which conducts a two-dimensional scanning of an object with a light beam so as to record or read an image and, more particularly, to a two-dimensional image scanning apparatus suitable for use in, for example, the medical field in which the requirement for accuracy of image recording and reading is specifically high.
2. Description of the Related Art
Hitherto, a two-dimensional scanning apparatus has been known which conducts a main scanning by oscillating a light beam unidirectionally by means of an optical scanning system while conducting sub-scanning by moving the scanned surface in a direction substantially perpendicularly to the direction of the main scanning. In this type of scanning apparatus, a constant scanning pitch cannot be obtained unless the sub-scanning is conducted at a constant speed, even if the main scanning is performed at a constant period. In fact, the quality of the recorded or read image is often impaired due to fluctuation in the movement of the scanned surface. It is said that even about 0.1% fluctuation in the pitch of the main scanning line is visually recognized and, hence, cannot be accepted when there is a strict demand for high image quality.
Various methods have been proposed to reduce movement fluctuation for the purpose of attaining higher accuracy of the pitch of the main scanning. For instance, Japanese Patent Examined Publication No. 62-11326 discloses an optical beam scanning apparatus in which, as shown in FIG. 7, a beam 102 from a laser oscillator 101 is modulated by means of an acousto-optical (A/0) modulator 103 and is deflected by a light deflector 104 so as to effect main scanning, while a table 106 held by guide bars 105 is fed by means of feed screws 107,108 so as to effect sub-scanning, wherein fluctuation in the sub-scanning speed is reduced in the following manner. Namely, a linear encoder 110 having a high precision or resolution is fixed on the table 106 and the movement of the linear encoder is photoelectrically detected to produce a signal each time the table 106 has moved a predetermined distance. The light deflector 104 for effecting the main scanning is operated in synchronization with this signal. According to this method, the precision of the pitch of the main scanning is determined by the resolution of the encoder, regardless of the precision of the mechanical feed of the table. It is thus possible to conduct the sub-scanning with a high precision by enhancing the resolution of the linear encoder.
This known scanning apparatus employs, as the light reflector, a sonic-optical deflection element, an electro-optical deflection element or an oscillating mirror such as a galvano-mirror. An oscillatory mirror such as a galvano-mirror is most stable when it oscillates at its resonance frequency. Therefore, when a galvano-mirror is used in the above-described optical scanning apparatus, the galvano-mirror tends to fail to operate with good response to the signal which is generated per unit movement of the table, with the result that the deflected beam cannot scan an original on the table at a constant speed. Such a variation in the main scanning speed causes defects such as local thickening or winding of the main scanning line, thus causing an uneven main scanning. On the other hand, a sonic-optical light deflecting element and an electro-optical light deflecting element can provide only a small deflection angle of 2.degree. to 3.degree. and, hence, are not suitable for reading or recording a large image.
FIG. 8 shows another proposed arrangement for reducing fluctuation in feeding speed. In this arrangement, a sub-scanning roller 206 is driven by a sub-scanning motor 209 through a belt 208 which is stretched between a pulley 207 fixed to the sub-scanning roller 206 and a pulley 210 fixed to the shaft of the sub-scanning motor 209 and which is capable of transmitting the torque of the motor from the pulley 210 to the pulley 207 while reducing the speed. Numeral 211 denotes nip rollers for pressing the recording sheet onto the sub-scanning roller 206 so as to stabilize the recording position of the recording sheet, 205, 212 denote guide plates, 204, 213 denote feed rollers which are arranged at positions where they do not engage the recording sheet when recording is being conducted on the recording sheet. An optical scanning system, (not shown) is capable of scanning the recording sheet with a light beam indicated by L. In this apparatus, fluctuation in the feed speed is reduced by the same principle as that used in known systems for driving a turntable of a record player. Namely, the sub-scanning roller 206 with a good balance of mass is driven by a motor 209 through a speed reducing transmission means the soft and flexible belt of which well absorbs jitter of the motor.
The use of the soft and flexible belt 208 for driving the sub-scanning roller 206, however, has a drawback in that the precision of rotation of the sub-scanning roller tends to be adversely affected by external factors such as vibration, change in the load acting on the sub-scanning roller, and so forth. Therefore, a specific measure is required such as positioning of the feed rollers 204,213 so as not to engage the recording sheet during recording, and so forth, resulting in an increase in the size of the apparatus.